from The Century Dictionary and Cyclopedia

Etymologies

(American Heritage® Dictionary of the English Language, Fourth Edition)

under +‎ croft (Wiktionary)

Examples

In the space known as the undercroft, classrooms equipped with low plastic banquet tables and tiny chairs are littered with toys, crayons, and construction paper, as volunteer teachers prepare activities and lessons for their two- to twelve-year-old charges.

But alas, now split between the glassed-off fortress of the Eurostar showroom and the far-off extension for the spurned Midland platforms, the undercroft filled with a standard-issue transport shopping mall, and enhanced with a Betjeman-themed pub, St Pancras has been multiplexed.

Maybe the staff control access to the undercroft because it's fairly cramped - you wouldn't want crowds of people trying to shove their way around down there - or maybe for the ubiquitous 'security reasons'.

A young man of about Cuhelyn's years, and by no means unlike him in build and colouring, though thinner in feature and somewhat longer in the reach, had been standing a little apart, watching the bustle all round him, his arms folded and his shoulders leaned against the wall of the undercroft, as though this tumultuous arrival concerned him rather less than the rest of the household.

A woman with her jutting belly printing the line from her breasts down along her navel to the undercroft of her belly through a tight pale blue dusty t-shirt looks blankly at us as she waves the green and red flags.

At the end of the service, the Archbishop carried the Blessed Sacrament in procession with the Dean and Chapter of the Cathedral to the Altar of Repose in the cathedral's undercroft to observe a vigil until midnight.